The present invention relates to a method for obtaining CO.sub.2 and N.sub.2 from the gases generated in the operation of an internal combustion engine, or turbine.
CO.sub.2 is normally obtained as a by-product in processes for producing ammonia, gasolines, fermentations and carbonate decompositions, although CO.sub.2 purification is difficult and costly.
In the industrial procedure of making liquid carbon dioxide one is able to distinguish five well defined phases crude gas generation, purification, compression and liquefaction, drying and rectification distillation.
In the generation phase, use is made of liquid fuels such as fuel-oil, or solid fuels such as good grade anthracites, coke, charcoal, etc., as raw materials, with care always taken for obtaining good combustion with excess atmospheric oxygen in order to make easier the complete oxidation of carbon: EQU C+O.sub.2 .fwdarw.CO.sub.2 +heat. (1)
Purification of the gases issuing from combustion involves effecting several essential treatments for the enrichment of CO.sub.2 until a volume concentration of 99.90% results. These treatments can be separate, as in: washing, absorption, desorption and stripping reducing substances off.
Washing is performed by means of a fresh water shower removing solids (soot, carried off ashes, etc.) and which at the same time cools the combustion gases and removes as well the sulfurous anhydride issuing from the sulfur contained in the fuel used for the generation. A second washing is effected with a Solvay soda diluted solution to reduce the primary gas contents to nitrogen, oxygen and CO.sub.2.
The washed gases are then caused to circulate through Raschig ring filled towers at the inside of which they meet with a counter-stream shower of absorbing solutions of potassium carbonate, monoethanolamine, etc. Potassium carbonate combines itself with one molecule of CO.sub.2 and one molecule of water, thus being converted into bicarbonate according to the following reversible reaction: EQU CO.sub.3 K.sub.2 +H.sub.2 O+.fwdarw.2CO.sub.3 HK. (2)
Pure CO.sub.2 desorption, or giving off, is effected by heating the CO.sub.2 saturated solutions over 100.degree. C. by taking advantage of the heat evolved in the combustion.
A last purifying treatment is performed as the CO.sub.2 passes through towers at the inside of which oxidizing solutions are made to recirculate, thus stripping off the traces of organic impurities eventually carried off with the gas.
In this manner, CO.sub.2 is ready for passing to the third, i.e. compression phase in which pressures in the range of 15 to 20 atm are reached by means of dry piston compressors. Thereafter the CO.sub.2 is cooled and liquefied by means of classical cooling networks, using ammonia, Freon, etc., lowering its temperature until the liquefying required pressure, is reached.
The thus obtained liquid CO.sub.2 has been previously freed from another impurity, the saturation water, most of which has already been removed in the intermediate and final coolers of the compression stage. Finally a strong drying is performed in special towers filled with highly deliquescent substances, which are regenerated again before they reach moisture saturation.
Finally, rectification has the purpose of eliminating the small amount of air gases (nitrogen, oxygen, argon) that had accompanied CO.sub.2 through all of the procedure. When this phase has been surmounted, the CO.sub.2 acquires a purity higher than 99.9% in volume.
Nitrogen isolating methods can be assembled in two classes: separation from the atmosphere, and nitrogenated compound decomposition. Industrial production methods consist of fractional distillation of liquid air.
Nitrogen with about 1% argon and traces of other inert gases can be obtained by chemically separating oxygen, carbon dioxide and water vapour from atmospheric air by means of suitable chemicals. The following chemical reactions have been used as well for the preparation of nitrogen:
When a saturated solution of sodium nitrite is mixed with a solution of ammonium chloride, the reaction is: EQU NH.sub.4.sup.+ +NO.sub.2.sup.- .fwdarw.N.sub.2 +2H.sub.2 O (3)
Ammonia, in gas form, is oxydized by passing it through bromine water, and the thus resulting gaseous mixture is thereafter separated by passing it through different reagents which absorb the unreacted bromine, water vapor and ammonia. The reaction is as follows: EQU 2NH.sub.3 +3BR.fwdarw.N.sub.2 +6H.sup.+ +6Br.sup.- ( 4)
Another method for obtaining nitrogen consists of heat reacting ammonia gas with metal oxides, for example: EQU 3CuO+2NH.sub.3 .fwdarw.3Cu+3H.sub.2 O+N.sub.2 ( 5)
The above are methods presently used for the isolation of CO.sub.2 and N.sub.2. Internal combustion engines or turbines do burn hydrocarbons to generate electricity, steam and/or hot water and combustion gases. Produced heat is generally used to take advantage of it, whereas the combustion gases are discarded.